Cathodes for fluorine gas discharge lasers

ABSTRACT

A fluorine gas discharge laser electrode for a gas discharge laser having a laser gas containing fluorine is disclosed which may comprise a copper and copper alloy cathode body having an upper curved region containing the discharge footprint for the cathode comprising copper and a lower portion comprising a copper alloy, with the facing portion of the electrode if formed in a arcuate shape extending into straight line portions on either side of the arcuate portion, the straight line portions terminating in vertical straight sides, with the boundary between the copper including at least the arcuate portion, the electrode may comprise a bonded element machined from two pieces of material the first made of copper and the second made of a copper alloy bonded together before machining. The electrode may also comprise a first and a second elongated lopsided V-shaped groove formed along substantially all of the elongated electrode body forming a discharge receiving ridge between the first and second lopsided V-shaped grooves, with a differentially faster eroding material filling the first and second lopsided V-shaped grooves. also disclosed is an electrode system in which the one electrode, e.g., the cathode bows during operation and may comprise at least one of a first and second elongated gas discharge electrode being machined to form a crown to receive the gas discharge that compensates for the bowing of at least one of the gas discharge electrodes during operation of the fluorine gas discharge laser.

[0001] This application is a continuation in part of U.S. patentapplication, Ser. No. 09/953,026, filed on Sep. 13, 2001, entitledDISCHARGE LASER WITH POROUS INSULATING LAYER COVERING ANODE DISCHARGESURFACE, with inventors Morton, et al., published on May 2, 2002, Pub.No. US20020051478 A1; U.S. patent application Ser. No. 10/081,589,entitled ELECTRIC DISCHARGE LASER WITH TWO-MATERIAL ELECTRODES, filed onFeb. 21, 2002, with inventors Morton et al., published on Oct. 24, 2002,Pub. No. US2002015467A10; U.S. patent application Ser. No. 10/104502,entitled HIGH REP-RATE LASER WITH IMPROVED ELECTRODES, filed on Mar. 22,2002, with inventors Morton, et al., published on Dec. 19, 2002, withPub. No. US20020191661A1; U.S. patent application Ser. No. 10/629,364,entitled HIGH REP-RATE LASER WITH IMPROVED ELECTRODES, filed Jul. 29,2003; U.S. patent application Ser. No. 10/638,247, entitled HIGHREP-RATE LASER WITH IMPROVED ELECTRODES, filed Aug. 7, 2003; thedisclosures of all of the above being hereby incorporated by reference.

[0002] This case is also related to Attorney Docket Nos. 2003-0048,entitled “ANODES FOR FLUORINE GAS DISCHARGE LASERS,” and attorney DocketNo. 2003-0058, entitled “ELECTRODE SYSTEMS FOR FLUORINE GAS DISCHARGELASERS,” filed on the same day as this application and assigned to thecommon assignee of this application, the disclosures of which are herebyincorporated by reference.

FIELD OF THE INVENTION

[0003] The field of the invention relates to electrodes and electrodesystems for fluorine gas discharge lasers.

BACKGROUND OF THE INVENTION

[0004] The above referenced previously filed co-pending applicationsrelate to various aspects of electrodes, particularly for electrodesystems utilized in gas discharge lasers, and more particularly gasdischarge lasers utilizing a laser gas containing fluorine, referred toas fluorine gas discharge lasers. In addition U.S. patent applicationSer. No. 10/243,102, fled on Sep. 13, 2002, entitled TWO CHAMBER F2LASER SYSTEM WITH F2 PRESSURE BASED LINE SELECTION, with inventorsRylov, et al., published on Jul. 24, 2003, with Pub. No.US20030138019A1, U.S. patent application Ser. No. 10/210,761, filed onJul. 31, 2002, entitled CONTROL SYSTEM FOR A TWO CHAMBER GAS DISCHARGELASER, with inventors Fallon et al., published on Feb. 13, 2003, withPub. No. US20030031216A1; U.S. patent application Ser. No. 10/187,336,filed on Jun. 28, 2002, entitled SIX TO TEN KHZ, OR GREATER GASDISCHARGE LASER SYSTEM, with inventors Watson, et al., published on Jan.16, 2003, Pub. No. US20030012234A1, and U.S. Pat. No. 6,584,132,entitled SPINODAL COPPER ALLOY ELECTRODES, issued to Morton, on Jun. 24,2003 discuss various aspects of fluorine gas discharge lasers andelectrode requirements for such lasers as well as other laser life,particularly chamber life issues surrounding the operation of suchlasers.

[0005] It is well known, as the above references discuss that theenvironment for electrodes in a fluorine gas discharge laser is complexand severe. Increasing requirements for output laser power, resultingin, among other things, higher voltages across the electrodes, andhigher total power dissipated in the discharges over electrode life,exacerbating the severity of he gas discharge laser chamber environment.The need to increase pulse repetition frequencies well above 4000 Hz,and even up to double that repetition rate during pulse bursts, equallycauses problems in maintaining electrode lifetimes. The need for morepulses per burst and other well known and increasing severe demands onthe gas discharge laser electrodes, particularly in fluorine gasdischarge lasers has lead to and will continue to lead to demands forimprovements in electrode and electrode assemubly technologies. Some ofwhich are more specifically directed to cathodes, and/or their assemblyas part of the laser chamber and some more specifically to anodes and/ortheir particular assembly. The electrical, electromagnetic, physical andchemical influences on electrode lifetimes continually place challengeson the designs for electrodes and their interfaces with other parts ofthe chamber, including the gas discharge region between the electrodesthemselves. The present application addresses some of the above notedconcerns.

SUMMARY OF THE INVENTION

[0006] A fluorine gas discharge laser electrode for a gas dischargelaser having a laser gas containing fluorine is disclosed which maycomprise a copper and copper alloy cathode body having an upper curvedregion containing the discharge footprint for the cathode comprisingcopper and a lower portion comprising a copper alloy, with the facingportion of the electrode if formed in a arcuate shape extending intostraight line portions on either side of the arcuate portion, thestraight line portions terminating in vertical straight sides, with theboundary between the copper including at least the arcuate portion, theelectrode may comprise a bonded element machined from two pieces ofmaterial the first made of copper and the second made of a copper alloybonded together before machining. The electrode may also comprise afirst and a second elongated lopsided V-shaped groove formed alongsubstantially all of the elongated electrode body forming a dischargereceiving ridge between the first and second lopsided V-shaped grooves,with a differentially faster eroding material filling the first andsecond lopsided V-shaped grooves. also disclosed is an electrode systemin which the one electrode, e.g., the cathode bows during operation andmay comprise at least one of a first and second elongated gas dischargeelectrode being machined to form a crown to receive the gas dischargethat compensates for the bowing of at least one of the gas dischargeelectrodes during operation of the fluorine gas discharge laser.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 shows a graph of anode profile changes over Bp;

[0008]FIG. 2 shows a graph of cathode profile changes;

[0009]FIG. 3 shows a typical axial anode erosion profile;

[0010]FIG. 4 shows a graph of cathode and anode erosion rates;

[0011]FIG. 5 shows cathode discharge width change for differentmaterials;

[0012]FIG. 6 shows a graph of surface roughness vs. alloy type;

[0013]FIG. 7 is a graph illustrating cathode erosion;

[0014]FIG. 8 is a graph showing worn anode and cathode surfacemorphology v. the composition of the material for an anode;

[0015]FIG. 9 shows an illustration of cathode related surface chemistrychanges following exposure to a laser chamber gas discharge;

[0016]FIG. 10 shows an electrode system exemplifying differentialerosion;

[0017]FIG. 11 shows a bonded bi-metallic cathode;

[0018]FIG. 12 illustrates a diffusion bond;

[0019]FIG. 13 shows a prior art example of an anode;

[0020]FIGS. 14a-f illustrate an exemplary artificial reefing process;

[0021]FIG. 15 shows a possible electrode configuration;

[0022]FIGS. 16a-c show multi-metallic electrodes having improved andtunable thermal conductivity properties;

[0023]FIG. 17 shows a cross-sectional view of an electrode arrangement;

[0024]FIG. 18 shows a multi-segmented electrode;

[0025]FIG. 19 illustrates another cathode;

[0026]FIGS. 20a-c illustrate an anode and cathode system with improvedlong life discharge shape control capabilities;

[0027]FIGS. 21a-b illustrate schematically a reef templating process;

[0028]FIG. 22 shows the proportions of ally materials in brass thatallow or inhibit reefing;

[0029]FIGS. 23a-d illustrate a bowed electrode concept;

[0030]FIG. 24 illustrates a structure for reducing erosion at the endsof an electrode;

[0031]FIG. 25 illustrates a tilted crown discharge region electrode.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0032] Cathode erosion in fluorine containing gas discharge chambers forgas discharge lasers comprising a laser gas containing fluorine, e.g.,Krypton Fluorine (“KrF”) or Argon Fluorine (“ArF”) and molecularfluorine (“F2”) lasers (herein referred to a “fluorine gas dischargelasers”) is very different from anode erosion. Anode wear is dominatedby F⁻ ion erosion corrosion, while cathode wear is dominated by highcurrent ionized noble gas ion ablation. In ArF excimer laser chambers,e.g., applicants observed that cathodes erode much more slowly thananodes. Cathode erosion rates have been found by applicants to betypically less than 30% of anode erosion rates. For brass alloys,cathode erosion rates have been found to trend relatively linearly withthe Zn content in the alloy.

[0033] Applicants believe this linear trending with the Zn content ofthe alloy is due to a combination of the mechanisms of Zn vaporizationand preferential sputtering from the parent alloy. In contrast to anodecorrosion, cathodes do not naturally grow self-passivating metalfluoride layers. Therefore, it is not possible to rely on a naturallygrown “reef”—like coating to passivate the alloy surface and protect acathode from erosion. Due to energy and discharge stabilityrequirements, slow eroding alloys appear to outperform engineeredpassivated/coated systems, i.e., attempts to artificially grow ordeposit “reef”-like self passivating material on the cathode. Inaddition, cathode wear morphology (pitting and roughness changes duringuse) appears to impact the erosion rates of the opposing anode.Applicants have determined that there is a clear interaction of cathodeand anode regarding material erosion rates. Therefore, it is desirablefor both the anode and cathode to be a compatible pair for optimumfluorine gas discharge laser chamber lifetimes.

[0034] In general, single phase materials such as Ni, and low meltingtemperature brass alloys (e.g., Zn levels >20%) appear to be better foranode erosion rates. Cu—Zn cathode alloys containing less than 15% Znappear to wear more slowly than high Zn containing brass alloys, howeverthey appear to pit as cathodes, apparently then accelerating anode wear.In general, the ideal cathode material will both erode slowly and stayrelatively smooth over the chamber lifetime.

[0035] As part of applicants' efforts to prolong chamber lifetime, andmore specifically electrode life, in applicants' assignee's products,e.g., in 7XXX laser products, applicants have evaluated candidate alloysfor both long life anodes and cathodes. In addition, applicants haveinvestigated optimizations for composite cathodes (e.g., fordifferential erosion) manufactured, e.g., via bonding processes, e.g.,diffusion bonding of Cu alloys. A series of 2 KHz laser pulse repetitionrate segmented anode and cathode tests were run. In these tests,applicants investigated both anode and cathode corrosion rates, e.g., asa function of alloy composition. Older style fluorine gas dischargechambers were utilized, e.g., 6XX0 chambers as have been utilized inapplicants' assignee's 6XX0 products were built and run using segmentedanodes or cathodes. By using multi-component anodes and cathodes,applicants were able to make “apples to apples” comparisons of materialerosion rates and to investigate anode/cathode interactions.

[0036] Using baseline 2 KHz KrF and ArF laser chambers for purposes ofconducting the evaluations. In order to minimize electrode geometryeffects, the same anode and cathode shapes were used for all tests. AllArF chamber tests used the same cathode part as are used in applicant'sassignee's ArF products and an anode, also utilized in applicants'assignee's ArF products. The KrF chambers used a KrF cathode and asegmented anode shapes similar to those used in the ArF tests. For thesegmented cathode tests, a ArF brass (C26000) anode was used opposite acathode with 7 sample alloys fastened onto the C26000 substrate. Allchambers were tested using standard gas fills for applicants' assignee's6010A and 6010K products for a minimum of 2 billion pulses (“Bp”) at 2to 2.5 KHz gas discharge repetition rates on applicants' assignee'slaser frames. Segmented electrode tests were run at fixed voltage(,e.g., 1100 volts), and operated in an optics-free mode. After thetests were completed, electrodes were removed, photographed, and erosionwas measured via a NIST calibrated dial caliper (0.01 mm resolution).

[0037] In order to quickly investigate the effects of Zn, Cu, and Pb onelectrode wear, a variety of readily available commercial copper alloyswere selected by applicants for testing. Since alloy manufacturing wasnot under process control, all Cu alloy samples were heat treated andsent out for external chemical and metallurgical characterization. Thiswas to investigate what impurities, if any, were typically in these Cualloy systems. In general, Cu alloys are frequently recycled, thereforemetallic and dissolved gas impurity levels are not well controlled. AllPb containing test alloy samples were annealed at 900 F to avoidhot-shortness (melting of the tertiary Pb alloy phase, which can causevoid formation in the material), and non-Pb containing alloys wereannealed at 1200 F. Special Zn and Ni alloys were not annealed. Table 1.Below, summarizes the alloys investigated in the segmented electrodestudies. Total metallic impurities were measured via glow discharge massspectrometry (“GDMS”) by wt. High impurity values suggest extensiverecycling of an alloy. TABLE 1 Summary of Metal Alloys Tested in 2 KHzSegmented Anode and Cathode Chambers (TD094, TD095, TD98, TD103, TD106)ArF Tests KrF Tests Composition Impurities Alloy # (A or C) (A or C) (Wt%) Grain Size (PPM wt)  1 A and C A and C Cu-99 125 350  2 A and C A andC Cu-99, Zn-5 90 225  3 A and C A Cu-90, Zn-10 89 N/A  4 A and C A and CCu-70, Zn-30 50 350  5 A and C A Cu-60, Zn-40 45 554  6 A and C A Cu-89,Zn-8.7, Pb-1.6 40 >3500  7 A A Cu-61.5, Zn-37, Pb-1.5 >200 >1000  8 Aand C A and C Cu-61.5, Zn-35.5, Pb-3 15 >10000  9 A A Cu-61.7, Zn-38,Pb-.3 >200 >5800 10 A A Cu-59, Zn-38, Pb-3 >200 >700 11 A A Cu-76,Zn-15, Pb 6, Sn >200 >500 3 12 A A Pb-99.9 N/A N/A 13 A and C ANi-99.9 >50 N/A 14 (GM) A A Zn-91, Cu-5.5, Al-3.0 N/A N/A 15 A A Zn-91,Cu-1, Al-8 N/A N/A

[0038] The above candidate alloys were selected to investigate the rolesof Cu, Zn, and Pb on alloy erosion in both anode and cathodeapplications. Historical data had suggested that, e.g., free machiningbrass and cartridge brass alloys have been extensively used as chamberanodes and cathodes, therefore Cu, Zn, and Pb were chosen as criticalalloying elements for study by applicants.

[0039] The metallurgy of brass is not very complicated. As Zn is addedto copper, the material forms a 2 phase (banded) microstructureincorporating the Zn and Cu within the crystal grains. If Pb is added, a3 phase microstructure develops since it is not soluble in Cu and Zn. Pbinclusions develop in Pb containing brasses during annealing at grainboundary interfaces. The grain boundaries, e.g., for free machiningbrass, have roughly 20 micron length and width dimensions and the leadinclusions form roughly 5 micron pockets at the junctions of severalgrain boundaries.

[0040] In general, Zn can be safely increased to 42 Wt % in brass alloyswithout creating materials that are brittle and/or difficult tofabricate into components. Zn helps strengthen the Cu alloy by reducingslip planes in the Cu microstructure. It also increases the sputteryield and vapor pressure of the Cu alloy, while also reducing meltingtemperature. Since Zn is also more reactive with Fluorine than Cu, therole of zinc in electrode erosion is of great interest.

[0041] Applicants also investigated the effects of cathode wearmorphology on anode erosion, i.e., anode-cathode wear interactions.Applicants have been aware that cathode segments oppositeself-passivating alloys (“reefing alloys) appear to wear quickly, andthat anodes wear faster across from cathodes that pit. applicants havegiven these self-passivating structures the name “reef” because of thegeneral appearance of a coral reef formed on the substrate electrodematerial in the self-passivating process and the terms are usedinterchangeably in this application.

[0042] Segmented electrode tests are a way that anode cathodeinteractions can be investigated in realistic time frames. In order toquantify electrode surface changes, cathode samples were evaluated forroughness changes by a so-called “Pocket Surf” Federal Profilometer,e.g., made by Mahr-Federal Inc. of Providence, R.I., and chemicallyinvestigated by SEM-EDX analysis.

[0043] Applicants have also observed that electrodes wear preferentiallyfaster on the preionization tube side of a fluorine gas discharge laserchamber. In addition, the electrode ends tend to wear at a higher ratethan the bulk of the electrodes, e.g., from within about 3 inches from apoint at the end of the electrode at either end of the electrodes wheregas discharge ceases to occur between the cathode and anode. Therefore,to avoid end effects, applicants only measured electrode erosionstarting at 4 inches from each electrode end, across the entireelectrode longitudinal length between those points. Erosion data for allelectrode segments and samples was then compared based on millimeters oferosion per billion pulses (“Bp”). FIGS. 1-3, below, illustratenon-uniformity in electrode wear. FIG. 1 shows anode profile changesover Bp. FIG. 2 shows cathode profile changes. FIG. 3 shows a-typicalaxial anode erosion profile for a 4 KHz ArF laser over approximately 3Bp.

[0044] Anode and cathode erosion including discharge width data for asegmented cathode are shown in FIGS. 4 and 5. FIG. 4 shows local C26000cathode and anode erosion rates in mm/Bp for a 2.5 KHz ArF laseroperating at 100 V C₀ for approximately 2.3 Bp. FIG. 5 shows cathodedischarge width change for different materials for a 2.5 KHZ ArF gasdischarge laser operating at 1100 V C₀ for approximately 2.3 Bp. One cansee that pure Cu alloys appear to wear more slowly than Zn containingalloys. Pb-containing brass alloys, e.g., 6 and 7 in Table 1, and highZn alloys, e.g., 5 in Table 1, also demonstrate more discharge widening(ArF) during the same relative number of pulses. In general, lowestcathode wear rates were observed with low Zn alloys (0-30% Zn). Pbcontaining cathode alloys were observed to wear faster in ArF laserdischarges.

[0045]FIG. 6 shows surface roughness vs. alloy type measured by “PocketSurf,” which is a mechanical profilometer. The measurement was afterapproximately 2.3 Bp in a 2.5 KHz ArF laser at 1100 V. Regarding surfaceroughness changes, cathode surface pitting appears to increase anoderoughness and may accelerate the anode erosion rate. The anode, e.g.,with the alloy No. 4 from Table 1, in general, appears smooth incomparison to all of the cathode samples after 2 Bp. Anode roughness isless than 20 micro inches roughness average (“Ra”), in comparison tocathode roughness (90 to 180 micro-inches Ra),. Ni produced thesmoothest cathode and adjacent anode surface after erosion. Applicantsbelieve this is related to Ni being a single phase material with a highmelting temperature and forming a chemically weak fluoride. Ni does wearrelatively fast as a cathode, however Ni reduces opposite anode erosionby 50% when compared to brass alloys. This may be due, however, to otherthan the composition of the Ni, e.g., the surface roughness of the Niand relatively low opposite alloy 4 from table 1 anode erosion couldalso suggest that anode erosion is accelerated by cathode rougheningwith or without a Ni or mostly Ni cathode.

[0046] Electrode erosion rates for the 2 KHz segmented anode and cathodetests were generally in line with erosion rates measured from electrodestested in 4 KHz laser chambers. It appears that for ArF, repetition rateor duty cycle changes do not significantly change the anode or cathodewear mechanism. Comparison cathode erosion data for the other 4segmented anode tests is summarized below. Applicants tested alloys 1and 2 from Table 1 for cathodes apposite the segmented anodes. Cathodeerosion rates were similar to those measured in the segmented cathodetest presented above. In addition, low Zn containing cathodes could pit,potentially increasing anode wear rates. Pitting was far worse oppositeself-passivating anode samples. It appears that the anode passivatingcoating (reef-like coating) accelerates cathode wear in Arf discharges.Applicants do not fully understand the mechanisms for this, since thereef changes both electrode roughness and surface electrical impedance.In general, cathodes in ArF chambers wear at 0.03-0.05mm per billionpulses when the anode is noted. Applicants' assignee had been previouslyaware of a 9 Bp 4 Khz ArF laser chamber that demonstrated alloy 4 (fromTable 1) cathode wear rate of 0.03-0.04 mm/Bp when run opposite an alloy8 anode. Applicants also discovered that KrF cathodes tend to wear muchfaster than ArF cathodes, approximately 0.05-0.06 mm/Bp. This is about a20-30% higher erosion rate for KrF cathodes vs. ArF cathodes. Inaddition, cathode wear rates are accelerated much more in ArF forreefing (self-passivating) anodes. Reefing anodes appear to damage ArFcathodes much more so than KrF cathodes made out of Cu—Zn alloys.Average cathode erosion data for the 4 segmented anode chambers can beviewed in FIG. 7.

[0047] The presence of an anode reef appears to increase cathode erosionin corresponding regions for both ArF and KrF applications. Applicantsbelieve that the reason is that the reef causes spatially non-uniformelectric field and current density. This may be mitigated by engineeringmore uniform artificially created reefs as opposed to naturally createdreefs, as discussed in more detail below. For KrF reefing systems,cathode erosion rates are generally increased by 10 to 20% when comparedto a non-reefing system. For ArF, the reef impact on cathode erosionrates appears to be even greater, where cathode erosion rates areincreased by 30 to 45%. Increasing cathode erosion rates, however, maybe acceptable if the anode does not wear and the self-passivatingcoating does not impact laser performance, which, again, may be thesubject of so-called reef-engineering, as discussed below.

[0048] Cathode erosion in the KrF chambers is found to be higher thanthat in ArF chambers. This is believed to be due to the higher gasoperating pressure in ArF chambers and/or the fact that Argon ions arelighter than Krypton ions, both of which lead to lower sputteringyields.

[0049] There appears also to be an interaction between the anode and thecathode. The magnitude of anode erosion appears to depend not only onthe anode material composition itself (how it reacts with fluorine), butalso on cathode surface morphology changes during erosion. Anodes appearto wear more slowly when facing a cathode material that erodes smoothly.The problem for both ArF and KrF laser chambers seems to be that it isnot apparent what materials meet this criteria of slow wear, i.e., moreslowly than the presently used brass alloys as a laser cathode. Nickelwears fast and smoothly as a cathode. Segmented cathode tests havedemonstrated that the C26000 anode segment opposite this Ni cathode woreat 50% the rate of adjacent segments facing brass alloys. Anoderoughness appears to be about the same for most segments. FIG. 8 showsworn anode and cathode surface morphology v. the composition of thebrass for a C26000 anode in an ArF laser operated at 2.5 KHz forapproximately 2.3 Bp.

[0050] Optimizing electrode life requires selection of materials thatphysically wear as slowly as possible, e.g., so as to prevent dischargegap widening, and that remain relatively smooth and homogeneous. For ArFlaser chambers, applicants had been unsuccessful to date using naturallygrowing self-passivating alloys since they suffer from high rep rateperformance issues, e.g., they act as insulators reducing the dischargeenergy locally and overall. Therefore, for ArF, the anode was believedto be required to be non-self-passivating and the cathode to wear asslowly as possible without pitting or roughening. From applicants'testing, Ni alloy cathodes produced the slowest C26000 anode wear rates,while pure Cu alloys produced the lowest cathode wear rate. However, Niwears fast as a cathode alloy, therefore seeming to disqualifying it foran ArF chamber application. In a similar sense, using pure Cu alloycathodes accelerates anode erosion due to pitting. As one increases thealloy Zn content to reduce pitting, the wear rate of the cathodeincreases. Therefore, cathode optimization would seem to require anengineering tradeoff considering surface roughening of the cathode vs.absolute wear rate.

[0051] For Cu—Zn alloys, it is apparent that, as Zn content isincreased, cathodes wear faster. However, for Cu—Zn cathodes, thecathode wear rate is very slow compared to the anode wear rate. Thoughhigh cathode Zn levels increase erosion rates, there is still about adifference of 0.01 mm/Bp (20%) over a 40% range in Zn content.Therefore, using a 20-30% Zn alloy can yield a good combination of lowcathode wear rate and anode “friendliness.” FIG. 8 shows cathode wearrate in 2 KHz ArF laser chambers vs. Zn content. Pictures demonstratingcathode morphology are attached to the figure, helping demonstrate thatsome Zn is needed to suppress local non-uniform melting and pitting onthe cathode surface when naturally occurring reefing is the operativemechanism. Applicants estimate from surface chemistry changes measuredby SEM and XRF, that up to 15% of the surface Zn can be extracted by thedischarge prior to ablation of the material.

[0052] Cathode related surface chemistry changes following exposure tothe laser chamber discharge are illustrated in FIG. 9 for a 2 KHz ArFlaser at up to about 2.3 Bp. In general, applicants have observed thatthe cathode region exposed to the discharge is depleted of Zn by up to15% absolute. In addition to having a higher sputter yield than Cu,applicants believe that the Zn may also vaporize at cathode operationaltemperatures. Cathode erosion results in local Zn depletion of the brassin the discharge footprint region. Zn has a relatively high vaporpressure for temperatures >500 C. Therefore applicants suspect weightloss due to Zn vaporization also, e.g., when cathode surface is >500° C.Visible melting of Cu and the presence of non-equilibrium PbF₄ deposits,as seen by XRD Analysis, on KrF cathodes (PbF₄ only being stable between300° C. and 800° C.) suggests to applicants that a cathode surfacetemp >500° C. is realized in operatoin.

[0053] It may be difficult to decouple sputter and vapor pressureeffects, since they would also suggest that Zn is preferentially removedfrom the brass. Regardless of what mechanism, it appears to applicantsthat there is a need for Zn at the cathode surface to suppress cathodepitting. Since cathode wear morphology appears to stabilize with Znlevels greater than 20%, applicants suggest using brass alloys withgreater than 25% Zn as cathodes is one approach for optimum electrodelife. Anode surface chemistry was observed to match the bulk electrodechemistry. Since the anode wears much faster than the cathode (alsoapparently coming from chemical attack) this suggests that Zn depletionon the cathode surface requires some time to develop. Assuming this tobe the case, a strong argument can be made that thermally induced Znevaporation may be occurring at the cathode surface.

[0054] In general, brass alloys outperform pure metals as chambercathodes. It is not completely clear why brass erodes so slowly as acathode in fluorine gas discharge laser chambers, however applicantsbelieve that the reactivity of Zn with F₂ gas (forming ZnF₂ solids) mayhelp slow cathode erosion. Theoretically, the ideal cathode materialwould consist of: 1) a single compositional element, 2) a singlecrystalline phase, 3) a low sputter yield element, 4) a large grainsize, 5) a low vapor pressure, 6) low metallic and dissolved gasimpurities, 7) the generation of solid reaction bi-products whencorroded by ionized fluorine gas.

[0055] As metals go, Cu and Ni appear to almost fit this profile,however neither forms an entirely successful cathode. Pure Cu appears topit, and consequently not be anode “friendly,” and Ni wears relativelyvery fast, though both also have alloys that demonstrate some of theabove positive traits of a cathode. As predicted, Cu wears slowly as acathode, however pitting appears to be tied to Zn content. Therefore,pure copper is not a reasonable cathode material since it increasesanode wear rates. Zn additions to Cu (brass alloys) reduce the meltingtemperature of the alloy, perhaps allowing some surface material reflowto cover up pit-like defects. Zn is very reactive with Fluorine, andtherefore it may also play a more chemical role in cathode pitsuppression. Ni, on the other hand, wears smoothly as predicted (i.e.,is anode “friendly”), but faster than is desirable. This may be due tothe poor reactivity of Ni with Fluorine gas. Ni does not pit as acathode material, suggesting that cathode pitting is not related tosurface chemistry effects (i.e., Zn is not needed to suppress pitting).Perhaps it may not be possible to find a material with all of the abovetraits. Brass is inexpensive, and wears at a consistent and predictablerate as a cathode, and appears not to pit in the case where the contentof the Zn is >20% (percent by weight).

[0056] Brass cathode pitting can be tuned by adjusting alloy Zn content.Applicants suggest that Zn additions should be held to a minimum sincecathode erosion is accelerated if high Zn levels are used. Perhaps forself passivating alloys, using an alloy 5 cathode may resist cathodepitting damage better than alloy 4. In addition, Pb additions to brasscathode alloys appear to increase wear rates and cathode roughening. Pbis not uniformly distributed in brass, and has the unusual ability toprevent grain growth. The Pb is relatively uniformly distributed in thebulk brass alloy, however, by non-uniformity is meant that, since, e.g.,the Pb is not very soluble in the alloy it segregates and “clumps” atgrain boundaries in different regions, so that, at, e.g., 500×magnification it can be seen that the distribution is very non-uniform.Pb is also highly reactive with fluorine. Pb may accelerate cathodeerosion by increasing fluorine ion surface affinity to the brass (i.e.,may be struck during ringing in the discharge). Pb also pins metalgrains, therefore preserving a fine grain structure. As sputter targets,fine grain materials erode faster than large grain materials since it iseasier to eject atoms along grain boundaries (i.e., reduce surfaceenergy). Since there are 2 possible mechanisms as to why Pb containingbrass alloys wear faster as a cathode than non-Pb containing alloys,applicants suggest using lead free brass alloys as a long life cathode(e.g., using alloys 4 or 5 from Table 1 or a C27000 brass).

[0057] It is also possible that, using the above principles to findcathode materials with various wear rates, one can combine 2 differentalloys with differing erosion rates to create differential erosioncathodes. A differential erosion cathode is illustrated in FIG. 10.Applicants have tried both Cu10200/alloy 4 and alloy 4/alloy 8 diffusionbonded cathodes. Both systems appear capable of exhibiting differentialerosion in KHz or ArF chambers.

[0058]FIG. 10 shows an electrode system containing a cathode 20 having amain body 21 made of a first material and an insert 21 made of a secondmaterial, with the first material 21 having a higher erosion rate thanthe second material 24, thus eventually forming a differential erosiontrench 24. It will be seen that the insert 24 eventual erodes away fromthe anode 22 over time as well. FIG. 10 also shows a cathode having amain body 23 made of a first material and an insert 26 made of a secondmaterial, e.g., one that will form a passivating layer (reef) 28 as ananode in a fluorine gas discharge laser. The differential erosion ratesof the first and second materials, aided by the reef 28, eventuallyforms an erosion trench 27 on either side of the insert 25 in the anode22.

[0059] A Cu10200/alloy 4 cathode is illustrated in FIG. 11, which hasbeen exposed to >2.5 Bp, at 4 KHz in an ArF laser chamber. One can seethat a step 26 has formed on the PI tube side of the cathode, the leftside as shown in FIG. 11, confirming that the alloy 4 is wearing fasterthan the C10200 (as predicted).

[0060]FIG. 12 illustrates an alloy 4/alloy 8 diffusion bond (Niinterlayer visible). From FIG. 12, one can see the large difference inmicrostructure between the annealed alloy 4 and alloy 8. Diffusion bondstrength for the Cu10200/Alloy 4 process was approximately 5000 PSI. Thesupplier Tosoh SMD was able to achieve bond tensile strengths of greaterthan 16,000 PSI (40-50% Yield strength) for the alloy 4/alloy 8 system,e.g., due to using a metal intermediate layer, e.g., the Ni layer shownin FIG. 12. Applicants believe that this higher bond strength will beadequate for a chamber electrodes, e.g., cathode, applications asexplained further below.

[0061] While diffusion bonding is common in some other industries, e.g.,the physical vapor deposition (“PVD”), i.e., sputtering, targetindustry, applicants are not aware that it has been used for theformation of multi-material, or the like, electrodes, and specificallynot for fluorine gas discharge electrodes, and further specifically notfor bimetallic electrodes that better maintain shape over life and/orfor differentially eroding structures. More specifically, applicants arenot aware that such alloys as alloys of brass, e.g., C36000 and C26000had been diffusion bonded before applicants suggested this be done, andmore specifically be done for electrodes, and specifically forelectrodes in a fluorine gas discharge laser environment.

[0062] Applicants company approached the above referenced vendor, ToshoSMD, of Grove City, Ohio to fabricate electrodes of the type variouslyreferenced in the present application utilizing diffusion bonding. Thevendor first created several prototypes, first using a typical Al to Tidiffusion bonding process. The vendor proposed utilizing a then knowntechnique for increasing tensile strength in the bonded materials whileotherwise preserving and even enhancing the structure of the bondedmaterial and also preserving the required alloy microstructures of thebonded materials. The vendor proposed forming an intermediate adhesionmetal layer between the metal being bonded. This layer may be formed,e.g., by coating each of the materials to be bonded at the bondinterface, which are thereafter subjected to the diffusion bondingprocess. The surfaces of the respective materials at the diffusion bondinterface may be roughened prior to such coating and the coating may bedone by a number of thin film or thick film coating techniques well knowin the art. This process is quite common for joining materials, asdiscussed in T. Nguyen, “Diffusion bonding—an advanced material processfor aerospace technology”, Aerojet, Sacramento, Calif.,http://www.vacets.org/vtic97/ttnguyen.htm.

[0063] The resultant bimetallic electrode exhibits bond strengths overother well known bonding techniques on the order of some 4×, while atthe same time preserving required alloy microstructures and otherproperties essential to proper performance as electrodes in a fluorinegas discharge laser environment. The vendor proposed a Ni adhesion layercoating, which has proved particularly successful in achieving resultsthat are desirable, e.g., for electrode applications for fluorine gasdischarge lasers. Applicants also believe that this technique of bondingcan be employed in making electrodes for the next generation laserlithography light sources, e.g., EUV electrodes, e.g., as platform or,e.g., Cu or Ni to which is bonded a hollow, e.g., generally cylindricalWolfram (“W”) or Wolfram-Thorium alloy (“W-Th”) EUV source, e.g., for aplasma focus source.

[0064] This has proven effective for the manufacture of multi-metallicstructures of different brass alloys and relatively pure Cu and a brassalloy.

[0065]FIG. 13 shows a prior art example of an anode 80, having an anodeblade 82, made of a metal in roughly a blunted blade shape, having afront side (in the direction from which laser gas is moved past theanode 80) with a front side fairing 84 and a rear side fairing 86. Eachof the front and rear side fairings is made, e.g., of a ceramicinsulator. The entire assembly is mounted on an anode mount 88.

[0066] Based on electrode erosion data, one can determine the etch rateratios between the lower wearing center element and the high wear rateouter material in a differential erosion cathode. In general, one wouldlike to select a material for the outer part of the cathode that wearsfaster than the center. Care also must be taken to select a centermaterial that does not pit the cathode. Applicants have found that edgeto center (“E:C”) erosion ration (so-called “selectivity” of erosion)etch rate ratio of 1.17 is able to develop into a differential erosioncathode, i.e., the material in the center eroding more slowly. One cando better with Copper based alloys, however, by combining alloy 4 fromTable 1 and alloy 8 from Table 1 an E:C etch rate ratio of 1.6attainable. The optimum, anode “friendly,” combination of materialswould seem to be Ni201 and C26000, with an E:C ratio of >5.8. It is notclear what the optimum ratio is for E:C etch rates, however at this timeapplicants feel that the higher the ratio, the better. One possiblechoice is combining C26000 and C36000 for a low risk differentialerosion cathode. TABLE 2 Erosion Rate Ratios for ProposedDifferential-Erosion Cathodes Differential Erosion Cathodes Center EdgeSelectivity (E:C) C10200 C26000 1.175438596 Demonstrated to 2.6 Bp.Known to pit C26000 C36000 1.625 Tested > 11 Bp C22000 C360001.857142857 Ideal full brass (may pit) C26000 Ni201 5.875 Best, SafeC22000 Ni201 6.714285714 Best overall

[0067] From the above, applicants have concluded that Cu—Zn cathodealloys erode at about 0.05 mm per billion pulses in ArF chambers, ArFanode materials (C26000 typical) typically erode at 0.15 mm/Bp. Cathodeerosion rates in KrF chambers are about 20% higher than ArF chambers forsome materials. Cathode wear rates seem to scale linearly with chamberrepetition rates. Zn additions to Cu suppress cathode pitting whileslightly increasing cathode erosion rates. Ni alloys are very anode“friendly,” however wear much faster than Cu alloys as a chambercathode. Anode roughening or reef (self-passivation) formationaccelerates cathode wear. Pb addition can increase cathode wear rates,and is not recommended for long life cathode centers. Cathode pittingappears to increase anode wear rates. Alloy 4 from Table 1 is a goodoverall cathode material for ArF and potentially KrF chambers.Differential erosion cathodes appear to work well for both KrF and ArFapplications. Differential erosion cathodes appear to work well withsystems with E:C etch rate ratios >1.15. Differential erosion cathodeswill likely work well for the long life KrF chamber applications, withproper alloy selection. In general, using alloy 4 instead of alloy 8 issuggested for improving KrF cathode life.

[0068] The present application contemplates a number of utilizations ofalloy and composite alloy uses for electrodes and specifically fordifferential erosion rate materials, to control, e.g., electrode shapesafter wear, rates of wear, etc. These include the following. Applicants'propose the use of high Zn, high Zn alloys, pure aluminum or high Snalloys, in conjunction with, e.g., bonded, e.g., by diffusion bonding,to high Cu alloys, e.g., with greater than 80 percent Cu. The formerhigh Zn, Zn alloy, Al and high Sn alloys erode relatively quickly andform with the higher Cu alloys preselected and beneficial wear over thelife of the electrode. It will be understood that these and likedifferential wear patterns discussed in the application can beself-regulating. That is, as the higher erosion rate material, e.g., oneither side of a lower erosion rate material erodes to form a depressionon one or both sides of the lower erosion rate material, the dischargewill become more focused on the protruding lower erosion rate material,which will then erode at a faster rate until erosion in the highererosion rate material on the side or sides of the central portion beginsto differentially erode again.

[0069] The present application contemplates the use of a claddingprocess to clad pure Cu to a self passivating alloy. Applicants havedetermined that copper, e.g., pure copper will wear faster than anyself-passivating electrode.

[0070] It is also possible, as is shown in FIG. 15 to create a shallowtrench 50, e.g., of about 0.5 inches in maximum depth, on either sideof, e.g., the crown 52 of an electrode 54, which may, e.g., be of thesame material as the rest of the body of the electrode 54, and to fillthe trench 50, e.g., with, e.g., zinc, e.g., in the form of zinc alloyor pure zinc, e.g., in molten form. Thereafter, after the zinc/zincalloy cools the assembly can be machined to form the desired shape,e.g., with the crown 52 similar to the blade 82 in FIG. 13, with thetrench 52 and remainder of the electrode 54 replacing the fairings 84.86shown in FIG. 13. Alternatively there may be formed the cross-section ofFIG. 11, with the crown 52 in the position of the insert shown in FIG.11. Indium or alloys or pure indium can similarly be used in replacementfor the zinc for making anodes.

[0071] It is also possible to leave the just-mentioned trenches empty tocontain the discharge on the portion of the electrode on the crown52between the trenches. This could also be done with multiple trenchesflanking respective multiple extending discharge receptor protrusions inthe nature of multiple crowns 52. Materials, e.g., spray coatings orelectroless coatings of, e.g., pure Ni, Ni alloys, e.g., Ni—Cu alloys,Sn or Zn or their alloys may also be deposited in such trenches 50,e.g., by spraying, thus flanking the electrode crown 52, and thenmachining the whole into a final shape. A Damacene process may be usedin this instance. A Damacene process, e.g., may involve the depositionof a material on a relatively rough surface and the polishing down ofthe high points to give an inlaid finish.

[0072] Another material that can be used to flank, e.g., a dischargecrown could be gildcop, an alumina doped copper material useable as ahigh erosion rate material.

[0073] As shown in FIG. 19, applicants have also developed another formof what might be called a “solder sidewalk” or alloy sidewalk cathode inwhich the trenches 50 as shown in FIG. 15 may be replaced with rotated“V” shaped slots 60 in which the one side 60′ of the “V” lies inessentially the vertical plane and the other side 60″ lies at an angleto the horizontal. The respective one sides 60′ of the respective “V”'s60 together form the side walls of a crown portion 62 of the electrode,e.g., a cathode 64. The rotated “V”'s may be filled with a suitablefaster eroding material than that of the cathode 64, e.g., a materialcontaining Pb, e.g., a Pb solder, while the cathode 64 may be made,e.g., of oxygen free copper (“OFC”) copper, e.g., annealed to obtain amaximum or nearly maximum grain size, as such annealing is known in theart. The “sidewalks” formed by the “V”'s may be formed by placing thematerial for the sidewalks, e.g., the Pb solder in the rotated “V”'s inmolten form or melting it into the “V”'s. allowing it to harden and thenmachining it down to the contour of the cathode 64.

[0074] In the course of fabricating some of the electrodes discussed inthe present application above various elements could be, e.g.,fabricated separately and joined by various means, including, e.g.,bolting, cladding or bonding, e.g., diffusion bonding, and perhaps thencould be machined or further machined to form the desired end productshape. Bonding processes may include various ways of forming compositematerials, e.g., single piece electrodes formed by diffusion bonding,explosion bonding, cladding, ultrasonic welding, friction welding,galvanizing and the like.

[0075] An upper surface of an electrode containing high content of aparticular material, e.g., Zn, may be formed, e.g., by annealing, e.g.,certain high Zn content brasses, e.g., alloy 4, C27000 or alloy 8, e.g.,at temperatures of 1200° F., e.g., for 60 minutes, e.g., per inch ofthickness. Applicants have found that thereby relatively thick layers ofrelatively pure Zn, e.g., of between 0.005 and 0.010 inches are formed.Applicants refer to this as, e.g., a Zn oxidation process, whichsegregates the Zn to the surface of the zinc-containing alloy. Zn,having a high affinity for oxygen readily segregates to the surfaceforming ZnO₂ at the surface.

[0076] One form of passivation layer/reefing engineering that applicantshave discovered makes use of processes such as those utilized in themanufacture of tin nanowires using a vacuum infiltrated porousanodization of aluminum. Such techniques can enable the creation of asynthetically created reef layer made of, e.g. a metal fluoride, on anelectrode created with relatively controlled pore size and distribution,having such properties as a controlled and evenly distributed porosity,e.g., as an anode reefing layer. In this manner the barrier reef may beengineered for controlled chemical and electrical properties, e.g.,impedance, e.g., to control the surface charge on the reef to, e.g.,avoid arcing, and in this manner optimize corrosion/erosion resistanceand optimize impedance.

[0077] Utilizing such a technique can enable the growing ofself-passivation material in relatively columnar and relatively evenlydistributed structures, resembling, e.g., in one embodiment an orchardof tree trunks and in another a honeycomb with generally circularopenings and a surrounding latticework structure. This relatively evenlydistributed orchard of tree trunks/laticework develops naturally throughthe mechanism of the anodization process and the resultant treetrunks/laticework can be employed to limit the transmission of, e.g.,charged ions present in the laser gas discharge, through the openings tothe electrode surface while maintaining the electrical conductivity,though the “tree trunks” themselves, as oxides, are insulative. Thissame mechanism of blocking ion transport can be employed in theanodization process as to the reactants to control the growth andseparation of the “tree trunks”/holes in the latticework.

[0078] The uniformity of the growth and separation of the “tree trunks”may be enhanced and its control made more uniform by, e.g.,pre-texturing the alloy surface on which the artificially engineeredreef is to be grown. This can be done, e.g., as shown in FIGS. 14a-f. InFIG. 14a there is shown a substrate 30, which may be the surface of anelectrode. The surface 30 of the electrode is shown in FIG. 14b to havebeen grown a first anodization 32, which contains a plurality ofrelatively evenly distributed “tree trunks” 34 and pores 36.

[0079] In FIG. 14b there is shown that after removal of the firstanodization 32 the surface of 30 is left textured. This step is thenfollowed, as shown in FIG. 14c, with a second anodization 32′, which asis shown results in a more uniformly distributed set of tree trunks 34′and pores 36,′ and also with the vertical orientation of the pores 36′being more uniform. The formulation in FIG. 14c may be useful in its ownright as an artificial reef coating on the surface of, e.g., anelectrode. It may also be further enhanced for desired results by, e.g.,as shown in FIG. 14d, widening, e.g., the pores, e.g., by an etchingprocess. This may also be followed by or include, as shown in FIG. 14e athinning or removal of the barrier layer 40 at the bottom of the pores36′ and subsequently filling the pores 36′ with a conductive material,e.g., metal 42. This process may be implemented in-situ in an operatinglaser chamber, e.g., by reversing electrode polarities in the step ofremoving the first reef.

[0080] Oxides other than aluminum oxide, e.g., ZnO, SnO₂ and PbO, whichare semiconductors in nature, but mostly insulative and have arelatively lower sputtering erosion rate than pure metals or metalalloys can be used, e.g., on the anode, to resist fluorine attack,without also adversely impacting impedance. This is proposed byapplicants as a substitute to the naturally growing reefing, whichapplicants have found can have adverse impacts on, e.g., systemimpedance, e.g., at higher gas discharge pulse repetition rates, e.g.,over 2000 Hz.

[0081] As opposed to forming an insulator and metallically doping thelayer or mechanically or ionically drilling holes in the layer on, e.g.,an anode, applicants propose to grown an oxide layer over the surface ofan electrode formed of the material which also forms the oxide, in pureform or alloy form, to create the oxide layer. Some such materials inthe pure form may not be suitable for use as an electrode, e.g., pure Znor Pb, due to other reasons, e.g., mechanical strength. However, forsome alloys, e.g., CuZn, e.g., subjected to oxidation in, e.g., afurnace oxidation process, an ozone oxidation process, a plasmaimmersion ion implantation oxidation process, an 02 plasma surfacetreatment oxidation process or by annealing in an oxygen atmosphere, orthe like process, the ZnO will grow faster than the CuO or CuO₂. The ZnOis also a more stable molecule. Also, due to the thin film nature of thegrowth, the layer of ZnO and CuO or CuO₂ will be very dense and protectthe electrode surface from erosion, e.g., due to fluorine attack, e.g.,due to sputtering under fluorine ion bombardment, and at the same timenot impact impedance significantly. Normally such a thin film oxide,e.g. SiO₂ is an insulator, e.g., in integrated circuit manufacture, butnot at the thickness involved in the present application and at thevoltages across the electrodes in a gas discharge laser as discussed inthis application.

[0082] Surface texturing may also be applied by, e.g., grit blasting theelectrode surface. This may be, e.g., a substitute for the firstanodization step shown in FIGS. 14a and b.

[0083] Differential erosion electrodes, containing, e.g., low erosionmaterial on the crown of the electrode, generally where the dischargeregion will mostly be, and higher wear rate material flanking the lowerwear rate material on the crown, in order to maintain an electrode shapeprofile in the discharge region may be formed, as has been demonstratedby applicants in, e.g., 2 KHz ArF and KrF laser chambers, considerationsof such other properties of the electrodes, e.g., thermal properties,e.g., heat transfer coefficients and profiles may also be taken intoaccount.

[0084] Diffusion bonding and other ways to avoid, e.g., solder jointsmay also attribute to the proper and adequate fabrication of suchbimetallic or tri-metallic electrode bodies, achieving, e.g., withdiffusion bonding multi-metallic structures that are formed with dense,high strength bonds with excellent thermal conductivity. The bondingprocess, e.g., diffusion bonding, e.g., utilizing an intermediate verythin third metal, e.g., Ni, layer, as shown in FIGS. 11 and 12 can beutilized to engineer the profile of the multi-metallic electrodestructure to alter thermal and/or chemical properties of the electrodebody. As shown, e.g., in FIGS. 16a-d, electrodes may be engineered to,e.g., modify the heat transfer through the electrode, e.g., to itselectrode mount, in order to, e.g., modify the thermal interactionbetween the electrode and its electrode mount, e.g., to prevent crackingof the mount, which may often be a relatively brittle ceramic materialsubject to its own interior thermal stresses and to the, e.g., bendingof the metal electrode induced by temperature changes and/or of the gasdischarge laser chamber under pressure.

[0085]FIG. 16a shows a relatively “hot” electrode 70, e.g., a cathode,which is formed from a body 72 comprising a material, e.g., brass, witha relatively low thermal conductivity and a relatively low meltingtemperature, diffusion bonded with an insert 74, of a material, e.g.,Cu, having a relatively high thermal conductivity and a relatively highmelting point. FIG. 16b shows a “medium temperature electrode 70′, e.g.,a cathode, comprising a body 72 and an insert 74, forming the crown ofthe electrode, along with an additional insert 76. The body 72 may beformed of a material with a relatively low thermal conductivity andmelting point and the inserts 74, 76 may be formed of a material havinga relatively high thermal conductivity. The second insert 76 may serveto carry more thermal energy away from the insert 74 and the body 72 ofthe electrode and to focus the thermal energy flux more at the boundaryof the insert 76 and the electrode mount (not shown). FIG. 16c shows arelatively “cold” electrode 70″ in which the expanse of the secondinsert 76 is such as to leave only a shell of the electrode body 72formed of the low thermal conductivity and low melting point materialand to leave much of the cross-section of the electrode 70″ formed ofthe relatively higher thermal conductivity and melting point material76. FIG. 16d shows another embodiment of the “cold” electrode 70′″ inwhich another insert 78 is contained within the insert 76 as shown inFIG. 16c composed of a material that is of a relatively low thermalconductivity and relatively low melting point, e.g., a brass, which mayor may not be the same as the brass in the portion 72 of the electrode70 body, and may be positioned to engineer the thermal interactionbetween the electrode 70′″ and, e.g., current feed through/mountingbolts, which may be attached to the electrode 70′″ along itslongitudinal extension in the region of the additional insert 78.

[0086] In the manner described above, the thermal profile of anelectrodes 70-70′″ and, e.g., its interaction with such elements as itsceramic mount and metallic feed-throughs, along with the thermalinteraction of the electrode and feed through assembly and the ceramicmount/insulator may be better engineered. It will be understood thatvarious utilizations of the bonding techniques discussed in thisapplication, e.g., diffusion bonding, facilitates the fabrication ofthese multi-segmented electrodes and, as discussed, tuning their thermalproperties. However, this particular embodiment f the present inventionmay also utilize simpler mechanical bonding, e.g., bolting or screwingand achieve the benefits of the embodiment of the invention.

[0087]FIG. 18 shows a cathode manufactured utilizing diffusion bondingtechniques that provides for longer cathode life without modification ofexisting cathode size, shape or other geometries. FIG. 18 illustrates ablock of material 120 which may be formed of a bar 122 of relativelyvery low erosion rate material, e.g., pure or essentially pure copper,e.g., annealed at 1200° F., forming a material having, e.g., a very lowsputter rate material in a fluorine gas discharge laser environment.This bar 122 may be, e.g., bonded, e.g., by diffusion bonding, to a bar124 of, e.g., brass, e.g., C26000 or C36000 or the like brass alloys,e.g., annealed at 700° F., providing, e.g., good machining andmechanical properties. The latter may be of particular importance inregard to the attachments and sealing requirements between the cathode120 and, e.g., the main insulator in a fluorine gas discharge laser.

[0088] The two bars 122, 124 may then be bonded together by a variety ofbonding techniques as noted above, giving, essentially molecular typebonds, e.g., diffusion bonding, as described above, and which may alsobe, e.g., further enhanced by putting a thin layer of bonding catalyst,e.g., a metal adhesion layer, e.g., Ni to facilitate an even smootherand tighter diffusion bond 126.

[0089] Thereafter the electrode may be machined from the bonded bars,e.g., as the outline 128 suggests.

[0090] In this manner a bimetallic cathode can be created thatstructurally, thermally, mechanically and in other ways behaves as if itwere monolithic, but the upper portion is very wear resistant in thefluorine gas discharge laser environment and the lover portion has manybeneficial properties that a pure copper electrode would not possess.This gives a very low erosion rate electrode, e.g., a cathode, withoutthe need for reliance on differential erosion, e.g., with an insert orcrown at the discharge region.

[0091] Applicants, through observation of the behaviors of anodizedaluminum corrosion in fluorine gas discharge laser environments haveproposed to combine anodized aluminum with, e.g., fastback fairingtechnology to, among other things, reduce downstream arching. An exampleof this can be seen in FIG. 17. FIG. 17 shows a cross-sectional view ofan electrode arrangement 90, transverse to the elongated extension ofthe electrode pair 90, the electrode pair 90 comprising, e.g., a pair ofgas discharge electrodes comprising a cathode 92 and an anode 94.

[0092] The anode 94 may comprise, e.g., a front-side (in the directionfrom which laser gas is moved between the electrode pair 90) including afront side fairing 96 having an upper front-side fairing longitudinallyextending surface 98, extending longitudinally along the length of theanode 94. The apposing front side fairing bottom surface 100 rests,e.g., on an anode mount (not shown), attached to the interior walls ofthe chamber (not shown). The anode 94 may also comprise, e.g., a backside wall 102 and a front side extension 104, extending from the uppertermination of the front side fairing 98. Extending between the upperextension of the front side extension 104 and the upper extension of therear side wall 102 may be formed, e.g., a discharge region 106 of theanode 94. The discharge region 106 opposes generally the dischargeregion of the cathode 92, e.g., generally along the longitudinal axis ofthe cathode 92.

[0093] Abutting the rear side wall 102 may be, e.g., a rear side(downstream of the gas discharge laser gas flow between the electrodepair 90) having, e.g., a rear side fairing 110 having a rear sidefairing upper surface 112, which may extent slightly higher along therear side wall 102 than the intersection of the front side fairing 96and the front side extension 104. The rear side fairing 110 may,therefore, extend laterally a longer distance than the front sidefairing 96. The rear side fairing bottom surface 114 may also rest onthe anode mount (not shown).

[0094] The entire anode 94 may comprise, e.g., Al. The exposed surfacesof the anode 94, including, e.g., the front side fairing upper surface98, the front side extension 104, the discharge region 106 and theportion of the rear side wall 102 extending above the abutting rear sidefairing 110 may be coated with anodization of the material of the anode94, e.g., anodized aluminum, e.g., from about 0.5-2.0 mils thick. Thethickness may be selected (tuned) for the desired combination ofimpedance and erosion resistance. The surface may also be roughened,e.g., with a 30-80 micro-inch Ra abrasive Al blast, e.g., air propelledsand. Grit-blasting/roughening could also be accomplished with anon-silica containing media. The anode may also be fabricated as notedabove utilizing, e.g., an A1 1100-0 alloy of 99% commercial gradealuminum along with other Si free aluminum alloys.

[0095] The above-described anode may be especially useful in promotinglong life in fluorine gas discharge lasers, particularly ArF fluorinegas discharge lasers. This is particularly useful in applying thecombination of a blade dielectric and a monolithic anodized anode 94with a monolithic front side fairing 98, which the applicants have namedan “airstream” anode after the aerodynamic aluminum mobile homes of themid-twentieth century. The passivated long life, e.g., anodized, pure oressentially pure aluminum anode with the “airstream” front fairing 98combined with the rear fairing 110 of a known blade dielectric anode,e.g. as generally shown above in FIG. 13, and slightly modifiedaccording to the presently described embodiment, can have a number ofbeneficial effects.

[0096] It will be seen that the combination of the blade portion of theanode exemplified in FIG. 13 with the monolithic front side fairing 96of the above described embodiment can serve to make the anode 94 easierto manufacture. This is due to the fact that pure aluminum orsubstantially pure aluminum is a relatively soft material and the largermonolithic cross-section gives the structure more strength to withstandthe manufacturing processes, e.g., machining. Also, aluminum anodizationis a very controllable process and can be tuned to optimize anodedischarge behavior from an erosion standpoint and at the same timeprovide adequate impedance characteristics for proper discharge betweenthe anode and the cathode. Thus can be avoided the uncertainties ofnaturally grown anode electrode reefs and the F₂ chemistry problemsfound to be associated with naturally grown reefs. Such tuning providesfor longer life of the anode, e.g., in a fluorine gas discharge laser,e.g., an ArF gas discharge laser. Finally the just described anode isless costly overall to manufacture, and install, including cost ofmaterials and fewer numbers of parts, than, e.g., the blade dielectricanode illustrated, e.g., in FIG. 13. A further advantage is theaerodynamics of the monolithic anode 94—front side fairing 96combination, and the fact that leading edge modifications, e.g., for gasflow dynamics, may readily be machined without having to worry aboutmachining a ceramic material.

[0097] Applicants have demonstrated an electrode as described with purealuminum and a sulfuric acid anodization in a fluorine gas dischargelaser, e.g., a KrF fluorine gas discharge laser at up to 2 Bp withoutevident damage.

[0098] An electrode system that applicants believe to be beneficial,e.g., in shaping the discharge and maintaining the shape of thedischarge over electrode life comprises, e.g., an electrode system 130as shown in FIG. 20a. FIG. 20a shows a cathode 132 and an anode 134,with the anode 134 having imbedded therein, below the discharge region amagnet 136, e.g., a rare earth magnet, e.g. which can be purchased at—,e.g. RadioShack and cathode 132 has imbedded therein below the dischargeregion a similar magnet 138. The north and south poles of the magnets136, 138 are oriented oppositely so that a magnetic flux field is formedbetween the anode magnet 136 and the cathode magnet 138, encompassingthe gas discharge 146 between the cathode 132 and the anode 134. Thismagnetic flux field tends to confine the discharge in the horizontalplane to tend to keep it formed more or less directly between thecathode 132 and anode 134 directly over the discharge region of thecathode 132 and anode 134 and to prevent that discharge region fromspreading out laterally over chamber life. The electrons of the gasdischarge and ions as well will tend to be confined within the lines ofhighest electric and magnetic field, so that the magnetic field booststhe confinement factor.

[0099]FIG. 20b shows an arrangement where the anode 154 has a pair ofauxiliary magners 160 and 160 distributed on either side of the magnet158, all of which magnets 158, 160, 162 may be rare earth magnets. Thecathode 152 and the anode 154 continue to have the embedded magnets asshown in FIG. 20a, i.e., magnets 156 and 158, which serve the purposesdescribed above in relation to FIG. 20a. The auxiliary magnets 160, 162contained in the anode 154 serve to provide a horizontal tuning of theelectric field to go along with the vertical tuning provided by themagnets 136, 138.

[0100]FIG. 20c shows the magnets 136 of the anode distributed generallyalong the longitudinal axis of the anode by way of illustration, withthe same configuration being possible for the cathode magnets 138.

[0101] Another form of reefing engineer that applicants believe to be ofgreat benefit in producing both long lived cathodes and anodes involvesthe use of so-called reefing templates. As shown, e.g., in FIG. 21a anexample of a so-called positive reefing template is illustratedschematically. In a positive reefing templates system, an electrode,e.g., an anode 182 is shown schematically. The anode 182 may be, e.g., amaterial that does not ordinarily form a reef when used as an electrodein a fluorine gas discharge laser, e.g., as an anode in an ArF fluorinegas discharge laser, e.g., a brass alloy of copper, e.g., C36500. Havinga relatively low fraction of Pb, along with a relatively high level ofZn, C36500 does not reef well as an ArF anode, as applicants have foundand as can be seen, e.g., in FIG. 22. C36500 is nominally 60% Cu, 37% Znand 3% Pb. On the upper surface of the anode, as shown in FIG. 21a 2 aredeposited in a predetermined pattern a plurality of deposits 184 of amaterial that would push the alloy of the anode 182 into the reefingportion of the curve of FIG. 22, had a corresponding reduction in thecontent of Zn occurred, i.e., Pb. It will be understood that manypatterns for the positive reefing template are possible and may result,also along with the materials used, in reefs of differringcharacteristics. For example, the template could for adjoining squaresin a checkerboard fashion, circles surrounded by intervening spaces,polygons surrounded by other polygons with no deposit, squares orrectangles separated by a grid of elongated straight “pathways,” and thelike.

[0102] Thereafter, as shown in FIG. 21a 3 the Pb in the deposits 186 isdiffused under thermal treatment into the upper very thin reaches of thematerial of the anode 182 to form diffusions 184. It will be understoodby those in the art that the diffusion process will tend to altersomewhat the dimensions of the pattern formed by the deposits 184 andthe shapes possibly as well, i.e., the diffusions will spread in aminiaturized version of an ink blot on a piece of relatively porouspaper, which ink does at room temperature and diffusions do under highthermal stress. The remaining shape and dimensions of the diffusions 184will, however, still allow for the reefing engineering according to anembodiment of the present invention.

[0103] As shown in FIG. 21a 4 exposure of the anode 182 to, e.g., afluorine gas discharge laser environment, e.g., in a ArF laser chamber,either during normal operations or, e.g., burn in testing duringmanufacturing, will now grow a reef having essentially relativelyparticularly placed segments 186 separated by essentially relativelyparticularly defined spaces 188. The placement of the reef sections 186in relation to the respective “seed” diffusions 184, and the size of thespaces 188 have been illustrated schematically in FIG. 21a to show thatthe growth over the diffusions 184, as was the case with he diffusions184 themselves, is not entirely uniform in extension away from therespective boundary of the respective diffusion 184 or on respectiveopposing side boundaries of a diffusion 184. Therefore separations 188between neighboring reef regions 186 may not be uniform throughout thereef region on the anode. Applicants have also discovered that the reefregions 186 so produced are almost free of Pb, so that the Pb diffusions184 catalyze the formation of the fluoride of the reef regions 186,e.g., CuFl, but remain in the substrate of the electrode 182 under thereef regions 186.

[0104] Nevertheless, such engineered reefs may be engineered by properselection of deposition templates, i.e., sizes, shapes and patterns ofdepositions 184 and diffusion processes, for somewhat relativelyuniformly forming the diffusions 184, such the reef regions 186 can begrown with their position, shape and extent on the surface of the anode182, e.g., in the discharge region, sufficiently controlled. The extentand positioning of the reef sections 186 and spaces 188 may besufficiently engineered to provide the insulation from, e.g., dischargeand/or ion erosion and at the same time provide impedances in therequired tolerances to promote long lived effective electrodes, e.g.,anodes, e.g., for fluorine gas discharge lasers that are reproducible totolerances that can be effectively manufactured.

[0105] A similar process is illustrated schematically in FIG. 21b 1-4.Here an electrode, e.g., an anode 190 comprised of, e.g., C36500, hasdeposited on it deposits 192 of a material that if in greater content inthe alloy itself would inhibit reefing as can be seen in FIG. 22, e.g.,Zn. The Zn is then diffused to form diffusions 194 and then the reef isgrown having reef sections 196 and openings 198, except that now theopenings 198 generally conform to the locations of the diffusions 196 asopposed to vice-versa in the process of FIG. 21a. The same issues ofcontrollability and manageability of the reefing process as were thecase described above for the “positive” reef templating process of FIG.21 a exist. Applicants believe, however, that these are not detrimentalto the production, or perhaps more properly stated for reefs grownduring actual fluorine gas discharge laser operation, the causation, ofreefs with the desired dual characteristics of protection againsterosion and allowing proper discharge, i.e., maintaining sufficientlylow impedance.

[0106] Applicants also believe that the size of the grain boundaries andthe number of grain boundary intersections in the material of the anodeplays a part in this process, e.g., in forming interstices at grainboundary intersections into which the Pb or Zn depositions can migratein the diffusion process. Roughly 20 micron separations of grainboundaries in at least one dimension of the grain seem to promote theprocesses discussed above. In addition applicants have found that aroughly 0.004 inch wide “seed” of, e.g., Pb, can promote the growth of aroughly 0.02 inch wide growth of reef.

[0107] Another aspect of electrode life that has been observed byapplicants can be seen in viewing FIGS. 23a-d. FIG. 23a shows aschematic side view of a portion of a gas discharge laser chamber 200having a cathode 202 and a main insulator 204 attached to the top 206 ofthe chamber 200. During operation of a gas discharge laser the chamber200 is placed under relatively high pressure, e.g., of 3-4 atmospheres,and it is known that this causes to roof 206 to bow upward, taking themain insulator 204 and electrode (cathode) 202 with it. This is shownschematically and not to proportion, for illustrative purposes, in FIG.23b. This bowing, while only about 0.005 inches at its peak point, cancreate problems in main insulator 204 cracking, which are addressed inways not the subject of this application, but is believed to also causeelectrode wear problems. Applicants have observed higher electrode weartowards the ends of the electrodes of fluorine gas discharge lasers,e.g., cathodes 202, and the facing portion of the anode 208, which willbe understood may not be at the end of the anode 208, since the anode208 may extend longitudinally longer than the cathode 202. Over time,this causes the discharge to be less compact at this end region and thusless effective, resulting in the overall discharge along the electrodeson average being less effective. This, at least in part, contributes tothe need for raising the discharge voltage required across theelectrodes over electrode/chamber life, in turn causing more erosionalong the remainder of the electrode per unit time as chamber/electrodelive increases. Problems can also result from arcing at the ends of theelectrode with surrounding grounded elements other than the opposinganode, due to the expanded discharge shape, which is also undesirable.

[0108] Applicants have proposed a solution to this bowing problem by themachining of a relatively equal and opposite bow in the crown region ofthe electrode, e.g., the anode 208′, as illustrated schematically andout of proportion for illustrative purposes in FIG. 22c. Thus, when theelectrode 202 and main insulator 204 bow as shown in FIG. 22b, theresultant profile of the cathode electrode 202 vis-a-vis its gap withthe opposing anode 208, generally not subject to any bowing, will form auniform gap over the entire length of the discharge regions formed onthe cathode 202′ and anode 208′. That is, the discharge region 210 onthe cathode 202,′ while bowed concavely as shown in FIG. 23b, will nothave its separation from the discharge region 211 of the anode 208′varying significantly over the longitudinal length of the cathode 202and anode 208′ (any more than tolerances would have dictate had thecathode 202 not bowed as a result of the pressure in the chamber 200).Specifically the cathode 202 and anode 208′ will not be relatively moreseparated progressing from the ends toward the middle of the anode 208′and cathode 202. Thus the anode 208′, as shown in FIG. 23c is formed asa “peaked” anode. It will be understood by those skilled in the art thatthe cathode 202 could be machined as well with a “peak” in the centersuch that when the cathode 202 bows with the main insulator 204, theanode 208 will also remain relatively equidistant from the cathode 202′as shown in FIG. 23d.

[0109]FIG. 24 shows one end of an electrode assembly 220 adapted toreduce expanded electrode erosion at the ends of the electrodes, i.e.,at the end 226 of the cathode 224, and the facing region of the anode(not shown in FIG. 24.). In the embodiment shown in FIG, 24 there isalso shown the anode mount 230 and the ground connection 232 of theanode mount 230 to the top half of the discharge chamber 222. Inaddition within the electrode assembly 220 are positioned a plurality ofcurrent return tangs 234 connected, e.g., by welding, between thechamber top half 222 and the anode mount 230.

[0110] Applicants have discovered that modifying the inductance in theregion of the electrode assembly near the ends 226 of the cathode 224,e.g., lowering the inductance, e.g., by removing current return tangs234 in this area reduces the expanded erosion at the ends 226 of thecathode 224. As shown in FIG. 24, the three tangs closest to the end 226of the cathode 224 have been removed leaving connection stubs 240 on thetop half of the chamber 222 and 242 on the anode mount 230.

[0111] Applicants have discovered that removing the tangs 234 at leasback along the cathode 224 length to beyond the region of expandedelectrode end erosion reduces the end erosion effects. In FIG. 24, theconnection stub 240 furthest to the left along the longitudinal lengthof the cathode 224 is just beyond the extent of the expanded electrodeerosion along the longitudinal length of the cathode 224 and the firsttang 234 is approximately_cm beyond this extend along the longitudinallength of the cathode 224.

[0112]FIG. 25a shows a cross-sectional view of an electrode, e.g., ananode that has been exposed to gas discharged voltages producing gasdischarges in, e.g., an ArF gas discharge laser. The anode 250 is shownin transverse cross section to comprise a main body 252, havingcontained within the main body an insert 254 in the area of thedischarge region for the anode 250. The insert 254 may be, e.g., of adifferent material than the main body 252. The anode 250 may alsocomprise a front side portion 258 in the direction from which gas flowis directed over the anode and a rear side portion 260. The insert 254may have a crown area 256 where generally the discharge is confinedduring fluorine gas discharge laser operation.

[0113] Then anode 250 illustrated in FIG. 25a is one which has beenexposed to a large number of discharges after which applicants observedthat the crown area 256, which initially was generally flat on top, hadpreferentially erodes on the left-most side of the crown 256 forminggenerally a slanted profile, slanted to the horizontal by about 4.5°.

[0114] Applicants observed that this slant is toward the side where inthe particular fluorine gas discharge laser the discharges occurred ispositioned a preionizing tube (not shown), as is well known in the art,which extends longitudinally generally parallel to the longitudinallength of the anode. Applicants believe that the differential erosion isdue to asymmetries in the gas discharge on the side of the preionizationtube due to the presence of the preionization tube and its electricaleffects on the discharge, and have also observed discharge splitting inthe discharge which appear to be related, at least in part, to sharpcorner features and/or wear features on the electrode. Applicants haveobserved a similar differential wear on the cathode, though perhaps notas pronounced. applicants also believe that asymmetries in the dischargeon one side of the discharge, particularly the left side as viewed inrelation to the anode as shown in FIG. 25, cause or at leastsignificantly contribute to the observed differential erosion.

[0115] In an effort to improve electrode life, particularly anode life,and avoid the detriments of discharge splitting, applicants propose anelectrode as illustrated schematically in FIG. 25b. There is shown ananode 270. It will be understood that the anode 270 may be the insert,as shown with respect to element 254 in FIG. 25a, intermediate metalfront side and rear side portions, 258,260, or may be a blade, e.g.,blade 82, e.g., as shown in FIG. 13, intermediate a front side ceramicfairing 84 and a rear side ceramic fairing 86.

[0116] The anode 270, like the insert 254 of FIG. 25a and the blade 82of FIG. 13 has generally straight side walls 274, 276. According to anembodiment of the present invention the anode 270 may be formed having agenerally elliptical crown portion 272 which is shown to haveintersections with each of the respective sidewall portions 274,276along a radius of curvature and to be tilted, i.e., its long axis istilted to the horizontal by, e.g., about 4.5° in the direction oppositeto that of the differential erosion shown in FIG. 25a. In this manner,the anode 270 may be formed with a beveled top that is beveled away fromthe differential erosion side providing a curvilinear upper surface thatis raise more on the side where the differential erosion will occur,e.g., the side of the asymmetry of the discharge, e.g., the side of thepreionization tube (not shown) and also avoids sharp edges at theintersection of the top portion 272 with the sidewall portions 274,276and/or due to the insert having a flat top and extending beyond theupper surfaces of the front side portion 258 and/or the rear sideportion 260. such an anode, can then serve to promote dischargestability by promoting e-field uniformities. Those skilled in the artwill understand that the cathode may be similarly constructed.

[0117] The above-described embodiments of the present invention areintended only for explanation and illustration purposes and are not theonly embodiments in which the present invention may reside. Thoseskilled in the art will understand that many modifications and changesmay be made to the described embodiments without changing the intent andspirit of the present invention. For example, while anode and cathodehave been used for the electrodes respectively that are grounded andconnected to high voltage, in truth the cathode could be connected to ahigh negative or positive voltage and thus in one case be a cathode andin another an anode, electrically speaking, with respect to the groundedelectrode, herein called in this context the anode. Similarly, whateverthe electrical power configuration, typically one electrode, hereinreferred to as the cathode, is mounted to the housing of the fluorinegas discharge laser, which historically is grounded, and the cathodehistorically has been the high voltage electrode, and thus must beinsulated from the chamber, e.g., the top of the chamber. Those skilledin the art will understand that it is possible to not have the chamberat ground, though not likely, and the aspects of the present inventionthat relate to modifications influenced by or dictated by theinterconnection of what is herein called the cathode and the insulatingmechanism, the main insulator, are not limited to the particularelectrode actually being electrically the cathode. Similarly thefluorine chemistry and the different interactions with fluorine and,e.g., the formation of reefs naturally and the impact of artificiallyforming reefs are independent of the denomination of the particularelectrode as a cathode or anode. They are dictated by current flow andion attractions. The use of cathode or anode in this application and inthe claims will be understood to be a convention common in the industrytoday with the grounded electrode in electrical contact with thegrounded chamber housing being referred to as the anode and the highvoltage electrode insulated from the housing being referred to as thecathode. However, equivalents of the claimed invention will beunderstood by those skilled in the art to exist where, electricallyspeaking, the anode of the present application is a cathode and viceversa. Other metals and alloys that have properties similar to thosediscussed in the present application may also form equivalentsubstitutes as those skilled in the art will appreciate.

I/we claim:
 1. A gas discharge laser having a laser gas containing fluorine comprising: a gas discharge electrode comprising: a copper and copper alloy electrode body having an upper curved region containing the discharge footprint for the electrode comprising copper and a lower portion comprising a copper alloy.
 2. The apparatus of claim 1, further comprising: the facing portion of the electrode is formed in a arcuate shape extending into straight line portions on either side of the arcuate portion, the straight line portions terminating in vertical straight sides, with the boundary between the copper and copper alloy including at least the arcuate portion.
 3. The apparatus of claim 1 further comprising: the electrode comprising a bonded element machined from two pieces of material the first made of copper and the second made of a copper alloy bonded together before machining.
 4. The apparatus of claim 2 further comprising: the electrode comprising a bonded element machined from two pieces of material the first made of copper and the second made of a copper alloy bonded together before machining.
 5. A gas discharge laser having a laser gas containing fluorine comprising: an elongated electrode body forming an arcuate or elliptical facing portion; a first elongated rotated V-shaped groove formed along substantially all of the elongated electrode body; a second elongated oppositely rotated V-shaped groove formed along substantially all of the elongated electrode body; the first and second rotated V-shaped grooves forming a discharge receiving ridge between the first and second rotated V-shaped grooves.
 6. The apparatus of claim 5 further comprising: a differentially faster eroding material filling the first and second rotated V-shaped grooves.
 7. The apparatus of claim 6 further comprising: the differentially faster eroding material is a solder.
 8. The apparatus of claim 5 further comprising: the differentially faster eroding material is a lead-tin solder.
 9. The apparatus of claim 6 further comprising: the differentially faster eroding material is a lead-tin solder.
 10. The apparatus of claim 7 further comprising: the electrode body comprising annealed copper.
 11. The apparatus of claim 8 further comprising: the electrode body comprising annealed copper.
 12. The apparatus of claim 9 further comprising: the electrode body comprising annealed copper.
 13. A gas discharge laser having a laser gas containing fluorine comprising: a first and a second elongated gas discharge electrode; the first and the second elongated gas discharge electrodes facing each other to form a gas discharge region between the first and the second elongated gas discharge electrodes: at least one of the first and second elongated gas discharge electrodes being machined to form a crown to receive the gas discharge that compensates for the bowing of at least one of the gas discharge electrodes during operation of the fluorine gas discharge laser.
 14. The apparatus of claim 13 further comprising: one of the gas discharge electrodes is a cathode attached to a wall of a gas discharge chamber of the fluorine gas discharge laser and the cathode bows during operation of the fluorine gas discharge laser; and at least one of the cathode and the other of the gas discharge laser electrodes is machined to maintain a constant gas discharge region separation between the cathode and the other electrode to accommodate for the bowing of the cathode.
 15. The apparatus of claim 14 further comprising: the cathode is machined to an apex at the center to accommodate for the bowing of the cathode.
 16. The apparatus of claim 14 further comprising: the other electrode is machined to an apex at generally the center to accommodate for the bowing of the cathode.
 17. A method of making an electrode for a gas discharge laser having a laser gas containing fluorine comprising: fabricating the electrode utilizing a copper and copper alloy cathode body having an upper curved region containing the discharge footprint for the electrode comprising copper and a lower portion comprising a copper alloy by diffusion bonding the upper curved region to the lower portion.
 18. The method of claim 17, further comprising: forming the facing portion of the electrode in a arcuate shape extending into straight line portions on either side of the arcuate portion, the straight line portions terminating in vertical straight sides, with the boundary between the copper and the copper alloy including at least the arcuate portion.
 19. The method of claim 17 further comprising: fabricating the electrode from a bonded element machined from two pieces of material the first made of copper and the second made of a copper alloy bonded together before machining.
 20. The method of claim 18 further comprising: machining the electrode from a bonded element formed of two pieces of material the first made of copper and the second made of a copper alloy bonded together before machining.
 21. A method of making a gas discharge laser having a laser gas containing fluorine comprising: forming an elongated electrode body in the shape of an arcuate or elliptical facing portion; forming a first elongated rotated V-shaped groove along substantially all of the elongated electrode body; forming a second elongated oppositely rotated V-shaped groove along substantially all of the elongated electrode body; the first and second rotated V-shaped grooves forming a discharge receiving ridge between the first and second rotated V-shaped grooves.
 22. The method of claims 21 further comprising: filling the first and second rotated V-shaped grooves with a differentially faster eroding material.
 23. The method of claim 22 further comprising: the differentially faster eroding material is a solder.
 24. The method of claim 21 further comprising: the differentially faster eroding material is a lead-tin solder.
 25. The method of claim 22 further comprising: the differentially faster eroding material is a lead-tin solder.
 26. The method of claim 23 further comprising: the electrode body comprising annealed copper.
 27. The method of claim 24 further comprising: the electrode body comprising annealed copper.
 28. The method of claim 25 further comprising: the electrode body comprising annealed copper.
 29. A method of making a gas discharge laser having a laser gas containing fluorine comprising: forming a first and a second elongated gas discharge electrode; the first and the second elongated gas discharge electrodes facing each other to form a gas discharge region between the first and the second elongated gas discharge electrodes: machining at least one of the first and second elongated gas discharge electrodes to form a crown to receive the gas discharge that compensates for the bowing of at least one of the gas discharge electrodes during operation of the fluorine gas discharge laser.
 30. The method of claim 29 further comprising: one of the gas discharge electrodes is a cathode attached to a wall of a gas discharge chamber of the fluorine gas discharge laser and the cathode bows during operation of the fluorine gas discharge laser; and at least one of the cathode and the other of the gas discharge laser electrodes is machined to maintain a constant gas discharge region separation between the cathode and the other electrode to accommodate for the bowing of the cathode.
 31. The apparatus of claim 29 further comprising: machining the cathode to an apex at the center to accommodate for the bowing of the cathode.
 32. The apparatus of claim 29 further comprising: machining the other electrode to an apex at generally the center to accommodate for the bowing of the cathode. 